Field of the Invention
The present invention relates to an image forming apparatus, an image forming system, and an image forming condition controlling method.
Description of the Related Art
Generally, an image forming apparatus (a printer, a copying machine, a facsimile, etc.) using an electrophotographic process technique forms an electrostatic latent image, by irradiating (exposing) a charged photoreceptor drum (image carrier) with a laser beam based on image data. Further, a toner is supplied from a developing device to the photoreceptor drum on which the electrostatic latent image has been formed, thereby visualizing the electrostatic latent image to form a toner image. Further, after the toner image is directly or indirectly transferred onto the sheet, the toner image is formed on the sheet by fixing the toner image through heating and pressurizing using a fixing nip.
Further, an image forming system, in which a sheet feeder which feeds continuous sheet (hereinafter, referred to as “long sheet”) such as continuous roll sheet or folding sheet, and a sheet discharge device which stores the long sheet with the toner image formed by the image forming apparatus are connected at a preceding stage and a succeeding stage of the image forming apparatus respectively, has been put into practical use.
When the sheet is conveyed from a sheet feeding unit or the like, if each sheet in such an image forming apparatus, for example, is charged by rubbing, the transfer efficiency fluctuates at a transfer nip serving as a portion in which the toner image is transferred onto the sheet.
In order to solve this problem, for example, JP 2004-69938 A discloses a configuration in which the transfer condition for each part of a sheet is changed based on the surface potential of the sheet before entering the transfer nip.
Further, JP 2005-274892 A discloses a configuration in which a predetermined bias is applied to a conveyance belt that conveys a sheet to adjust the potential of the conveyance belt, thereby adjusting the potential of the sheet conveyed on the conveyance belt.
Meanwhile, in the image forming apparatus compatible with the long sheet, the long sheet is set in the apparatus before the start of the image forming operation. When the image forming operation is started, the long sheet is printed sequentially from apart which is set in the apparatus. FIG. 1 is an enlarged diagram showing a portion wound in a roll shape in the long sheet.
As shown in FIG. 1, when the image forming operation is started, a long sheet P is peeled off from an unwinding position Z, located on the most upstream side in the rotary direction, of a surface of a roll portion PP, which is a portion wound in a roll shape. At this time, when the long sheet P is peeled off, the surface of the roll portion PP is charged due to, for example, influence of static electricity or the like. In particular, in a film type medium such as a film tack sheet on which different materials are laminated, the sheet is in close contact with the roll portion PP. Therefore, charging due to peeling occurs remarkably. Meanwhile, in the case of a sheet type medium such as a commonly used cut sheet, such charging does not occur because the medium is not peeled off.
At the time of start of the image forming operation, a portion set in the apparatus and a preceding stage portion which is a downstream portion of the unwinding position Z in a rotary direction of the roll portion PP are not charged. For this reason, when a succeeding stage portion, which is an upstream portion of the unwinding position Z, is peeled off from the roll portion PP and charged after the start of the image forming operation, the surface potential of the long sheet differs between the preceding stage portion and the succeeding stage portion. Accordingly, when printing is performed under the same image forming condition, there is a problem of changes in image quality between the preceding stage portion and the succeeding stage portion.
FIG. 2 is a diagram showing changes in the surface potential of the long sheet with respect to the peeling rate at which the long sheet is peeled off from the roll portion. For example, as shown in FIG. 2, because it is empirically known that the intensity of charging generated by peeling of the long sheet and the roll portion, that is, the magnitude of the surface potential depends on the peeling rate, the surface potential of the long sheet increases as the peeling rate increases. In addition, when the long sheet is unwound from the roll portion, it always moves away from the roll portion at a tangent angle or more. Thus, as the long sheet is unwound, the roll portion becomes smaller in diameter, and the peeling rate increases.
For this reason, at the time of continuous image forming operation, when the transfer on the long sheet is hindered by an increase in the surface potential of the long sheet due to an increase in peeling rate, the transfer efficiency decreases, and therefore, the image density of the succeeding stage portion of the long sheet becomes lower than that of the preceding stage portion.
Furthermore, when the long sheet is charged, in the halftone image formed of dots, the dot portions are scattered around or attracted to each other. As a result, image failure due to collapse of the dot shape occurs. For this reason, the density of the image varies between the preceding stage portion and the succeeding stage portion of the roll portion, and furthermore, when continuous printing is performed, a large difference occurs in the density between the image of the preceding stage portion of the long sheet and the image of the succeeding stage portion of the long sheet.
Further, in the case of a film tack sheet, due to the charging caused by a combination of different materials as well as the sheets having a smooth surface coming into close contact with each other at the portion of the roll portion, the amount of charge due to peeling further increases. Further, when overprinting is performed, charging is performed by transfer in base printing. As a result, the amount of charge on the surface of the long sheet becomes larger than that of the long sheet which is not subjected to overprinting. Further, the charged state also varies depending on the density of the image subjected to the base printing. Even in such a case, due to the influence of charging caused by peeling of the long sheet, a difference in image quality occurs between the preceding stage portion and the succeeding stage portion of the long sheet.
Further, the configurations described in JP 2004-69938 A and JP 2005-274892 A are techniques for solving a problem caused by frictional charging on sheets such as a cut sheet. Therefore, such techniques are not sufficient as a countermeasure against charging caused by peeling of a long sheet wound in a roll shape, and cannot solve the problem of charging caused by peeling.